ASDEX Upgrade | EPFL Graph Search

ASDEX Upgrade (Axially Symmetric Divertor Experiment) is a divertor tokamak, that went into operation at the Max-Planck-Institut für Plasmaphysik, Garching in 1991. At present, it is Germany's second largest fusion experiment after stellarator Wendelstein 7-X. Overview To make experiments under reactor-like conditions possible, essential plasma properties, particularly the plasma density and pressure and the wall load, have been adapted in ASDEX Upgrade to the conditions that will be present in a future fusion power plant. ASDEX Upgrade is, compared to other international tokamaks, a midsize tokamak experiment. It began operation in 1991 and it succeeds the ASDEX experiment, which was in operation from 1980 until 1990. One innovative feature of the ASDEX Upgrade experiment is its all-tungsten first wall; tungsten is a good choice for the first wall of a tokamak because of its very high melting point (over 3000 degrees Celsius) which enables it to stand up to the very h

Particle transport in tokamak plasmas

Alexey Zabolotskiy

The transport of particles in magnetically confined plasmas is of great importance for the development of fusion energy. It will determine techniques for fuelling, for controlling impurity concentrations and for the removal of the alpha particles produced by fusion reactions. The issues related to particle transport have received, until relatively recently, less attention than those regarding heat transport. Besides the greater experimental difficulty of measuring particle transport, this may explain why our understanding of this subject is still incomplete. The aim of this thesis is to document and tentatively interpret the experimental density profile behaviour in the TCV (Tokamak à Configuration Variable) and JET (Joint European Torus) tokamaks in the framework of different theoretical and semi-empirical models. The TCV tokamak is well suited to transport studies due to its extreme shaping capability, which allows the exploration of a wide range of different plasma conditions. This versatility is matched by the powerful and flexible electron cyclotron heating (ECH) system on TCV, which allows a control of the local power deposition profiles and current drive profiles. A study of particle transport on the JET tokamak has allowed us to compare the results of TCV to those of a much larger device and supplement the TCV study with the analysis in reactor relevant high confinement regime (H-mode). The experimental information was compiled into a database of density profiles in steady state, containing nearly 1000 samples for TCV and 600 samples for JET. The data analyzed covered a wide range of discharge conditions, including low confinement regime (L-mode) and H-mode discharges, ECH, ICH (ion cyclotron heating), LHCD (low hybrid), beam heated plasmas and include fully current drive discharges. The most relevant parameters which influence the density profiles were determined by regression. A detailed analysis of the particle sources showed that edge fuelling in TCV and JET cannot be responsible for density gradient in the plasma bulk, confirming the presence of particle convection or a 'pinch'. The existence of an anomalous pinch was unambiguously demonstrated both on JET and on TCV by the observation of peaked density profiles in stationary, fully relaxed, fully current driven discharges and hence in the absence of the neoclassical Ware pinch. An unexpected difference in the parameter dependencies was found in L-and H-modes. In TCV and JET, density gradient lengths (or profile peaking parameters) in L-mode were found to depend on magnetic shear with no dependence on collisionality. This lack of collisionality dependence in L- mode is inconsistent with current theoretical models. H-mode density profiles in JET, on the other hand, are clearly dependent on collisionality in agreement with theory and a prior observation on ASDEX-Upgrade, while exhibiting only a weak or no dependence on shear and temperature profiles. It was found that for TCV and JET, L mode density peaking can be interpreted as being due to turbulent equipartition, which assumes conservation of the magnetic moment and the longitudinal invariant during transport. The observation of a reduction of peaking in TCV with ECH supports drift wave turbulence theory, which predicts the appearance of outward particle convection, when trapped electron modes are destabilized. In JET H-mode, the weak secondary correlation of peaking with the electron-ion temperature ratio Te/Ti, may also be considered, at least quantitatively, as being supportive of drift wave turbulence theory.

EPFL2005

Model-based electron density estimation using multiple diagnostics on TCV

Federico Alberto Alfredo Felici, Cristian Galperti, Francesco Pastore, Olivier Sauter, Anna Ngoc Minh Trang Vu

Estimation of the dynamic evolution of electron plasma density during a tokamak discharge is cru- cial since it directly affects the plasma performance, confinement and stability, therefore it needs to be monitored and controlled. Knowledge of the density profile can also be used to control in a more direct way the desired aspects of the plasma density, for example choosing to control the core, volume averaged or edge density, replacing control methods that rely e.g. on a single line-averaged electron density from a specific interferometer chord. The reconstruction of the density profile can be performed with the RAPDENS code [1], employing the Extended Kalman Filter technique. The code collects the electron plasma density measurements from the available real-time diagnostics and uses them to constrain the solution of a predictive model that describes the 1D particle transport equation for the electron plasma density. Following recent improvements to the code for use on ASDEX-Upgrade [2], we report on the applica- tion of this method for routine reconstruction of density profiles in the TCV tokamak. In particular the transport coefficients of the electron diffusion equation, as well as the ionization depth of the fueling gas, have been tuned heuristically with the available off-line diagnostics data for higher fi- delity reconstruction of density profiles in L-mode, H-mode and negative triangularity discharges in TCV. We also present an extensive comparison of real-time estimated density profiles with the off-line Thomson scattering measurements across a wide range of experimental conditions. Finally, we report the first results on the upgraded detection and correction of interferometer errors with a model-based approach relying on the observer. References: [1] T. C. Blanken et al. “Control-oriented modeling of the plasma particle density in tokamaks and application to real-time density profile reconstruction”. In: Fusion Engineering and Design 126 (2018). issn: 09203796. doi: 10.1016/j.fusengdes.2017.11.006. [2] T.O.S.J. Bosman et al. “Kalman filter density reconstruction in ICRH discharges on ASDEX Upgrade”. In: Fusion Engineering and Design 170 (Sept. 2021). issn: 09203796. doi: 10.1016/ j.fusengdes.2021.112510.1

2022

Experimental Validation of a Filament Transport Model in Turbulent Magnetized Plasmas

Heinz Mueller, Nicola Vianello

In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices.

Amer Physical Soc2015